Recently, a laminated coil has been frequently used in electric and electronic fields because of its capability of size reduction and excellent mass-productivity. In this laminated coil, a plurality of insulating layers and a plurality of coil patterns are integrally stacked in a desired order. The coil patterns are connected to form a coil in the laminated body. In general, outer peripheral edges of the coil patterns are provided on the inner sides of outer peripheral edges of the insulating layers with a gap being disposed therebetween in a manner such as not to be exposed from an outer peripheral surface of the laminated body. The insulating layers are formed of a magnetic material or a nonmagnetic material.
It is known that the coil characteristic of the laminated coil can be improved by increasing the size of the coil patterns. For example, in a magnetic-core laminated coil including insulating layers formed of a magnetic material, the direct-current superposition characteristic of the coil can be improved by increasing the inner diameter and outer diameter of coil patterns while maintaining the same width of the coil patterns.
In contrast, in an air-core laminated coil including insulating layers formed of a nonmagnetic material, the Q-value of the coil can be increased by increasing the inner diameter and outer diameter of coil patterns while maintaining the same width of the coil patterns.
In the magnetic-core and air-core laminated coils, the direct-current resistance of the coil patterns can be decreased and the Q-value of the coils can be increased by increasing the width (outer diameter) of the coil patterns while maintaining the same inner diameter.
However, when the size of the coil patterns is increased in the laminated coils, the total size of the laminated body increases.
Accordingly, Japanese Unexamined Patent Application Publication No. 2000-133521 proposes a laminated coil that solves the above-described problems. In this laminated coil, the size of coil patterns is increased, but no gap is formed between outer peripheral edges of the coil patterns and outer peripheral edges of insulating layers so as to prevent an increase in the total size of a laminated body. The problem of the coil patterns being exposed from an outer peripheral surface of the laminated body is solved by forming an insulating film made of an insulating resin around the outer peripheral surface of the laminated body.
FIGS. 5 to 7 illustrate a laminated coil 300 disclosed in the above publication. FIG. 5 is a perspective view of the laminated coil 300, FIG. 6A is a cross-sectional view taken along chain line X-X of FIG. 5, FIG. 6B is a cross-sectional view taken along chain line Y-Y of FIG. 5, and FIG. 7 is an exploded perspective view of the laminated coil 300. In FIG. 7, external electrodes and an insulating film are not shown.
As illustrated in FIGS. 5 to 7, the laminated coil 300 includes a laminated body 103 formed by integrally stacking substantially rectangular insulating layers 101, formed of a magnetic material or a nonmagnetic material, and coil patterns 102 in a desired order. The coil patterns 102 have a large diameter, and outer peripheral edges thereof are entirely in contact with outer peripheral edges of the insulating layers 101. That is, there is no gap between the outer peripheral edges of the coil patterns 102 and the outer peripheral edges of the insulating layers 101. Further, the coil patterns 102 are connected by via-hole conductors 104a provided through the insulating layers 101, thereby forming a coil 105 in the laminated body 103. No coil patterns 102 are stacked near both ends of the laminated body 103, where a plurality of insulating layers 101 having via-hole conductors 104b, through which the coil 105 is led out, are stacked.
A pair of external electrodes 106a and 106b is provided at opposite ends of the laminated body 103. The external electrode 106a is connected to one end of the coil 105, and the external electrode 106b is connected to the other end of the coil 105. Further, an insulating film 107 formed of an insulating resin is provided around an outer peripheral surface of the laminated body 103. The insulating film 107 is provided to insulate the outer peripheral edges of the coil patterns 102, which are exposed from the outer peripheral surface of the laminated body 103, from the outside.
In the laminated coil 300 having the above-described structure, when the insulating layers 101 are formed of a magnetic material, the coil 105 is a magnetic-core coil. However, since the outer peripheral edges of the coil patterns 102 reach the outer peripheral surface of the laminated body 103, the coil 105 serves as an open magnetic circuit coil. Therefore, magnetic saturation is unlikely to occur, and the decrease in inductance is suppressed when direct current flows. This improves the direct-current superposition characteristic.
For example, the laminated coil 300 of the related art is produced by the following method.
To produce multiple laminated coils 300 together, a plurality of mother green sheets (not illustrated) are prepared and these serve as bases of insulating layers 101 and are formed of, for example, a ceramic material. Next, via-hole conductors 104a or 104b for the laminated coils 300 are formed in the mother green sheets, and coil patterns 102 are formed on the mother green sheets as required. For example, the via-hole conductors 104a and 104b are formed by filling holes preformed in the mother green sheets with conductive paste. For example, the coil patterns 102 are formed by applying conductive paste in a predetermined shape on surfaces of the mother green sheets by screen printing.
Next, the mother green sheets having the predetermined via-hole conductors 104a and 104b and coil patterns 102 are stacked in a predetermined order, pressed, and fired in a predetermined profile, so that a laminated body block including a plurality of laminated bodies 103 is formed. Then, the laminated body block is cut into a plurality of laminated bodies 103.
Next, an insulating film 107 is formed around an outer peripheral surface of each laminated body 103, and external electrodes 106a and 106b are formed on opposite end faces of the laminated body 103, so that a laminated coil 300 is completed. For example, the insulating film 107 is formed by applying thermosetting epoxy resin onto the outer peripheral surface of the laminated body 3 by dipping or printing and setting the epoxy resin with heat. For example, the external electrodes 106a and 106b are formed by dipping end portions of the laminated body 103 in conductive paste and baking the conductive paste applied on the end portions. Outer layers are sometimes further formed on the external electrodes 106a and 106b by plating.
Since the laminated coil 300 of the related art has the above-described structure, the size of the coil patterns can be increased to improve the coil characteristic without increasing the total size of the laminated coil 300.